Further analysis of the action of ibotenic acid: Experiments will be performed to evaluate if ibotenic acid induced neuronal degeneration is due to its ability to stimulate glutamate receptors in the brain. Further analysis of gabaergic and dopaminergic interactions in the brain: So far the analysis with putative gabaergic drugs, such as muscimol, have indicated existence of both excitatory and inhibitory influences on the ascending dopamine pathways. To further evaluate the specificity of the actions of muscimol, effects of other types of GABA agonists such as THIP (4,5,6,7-tetrahydroisooxazolol(5,4-c)pyridin-3-o1) will be evaluated on dopamine turnover in various forebrain regions. Further analysis of the interactions of enkephalin and endorphin immunoreactive neurons with central catecholamine systems: Intraventricular and intravenous injections of various doses of synthetic enkephalin analogues and of alpha-, gamma- and beta-endorphin will be studied on catecholamine turnover and levels, especially in the subcortical and cortical limbic areas. It will also be tested if the opiate antagonist, naloxone, can counteract the changes in CA turnover induced by the various opioid peptides. Further analysis of the mechanism of action of antidepressant drugs: Prolonged treatment with various types of antidepressant drugs will be performed to evaluate if a 5-HT receptor supersensitivity development occurs at the postsynaptic 5-HT receptors following such treatment, using the radioligands 3H-5-HT and 3H-d-LSD. Further analysis of the mechanism of action of antipsychotic drugs: Prolonged treatment with cataleptogenic neuroleptic drugs, such as chlorpromazine and haloperidol and with noncataleptogenic neuroleptic drugs, such as 1-sulpiride and tiapride will be performed. Behavioural signs of development of dopamine receptor supersensitivity will be correlated with changes in dopamine turnover and in dopamine receptors in various forebrain regions, using as radioligands 3H-spiroperidol and 3H-ADTN.